1. Field of the Invention
The present invention relates to information reproduction apparatus, and in particular, to information reproduction apparatus and methods using rewriteable optical disks such as, for example, MO (magneto-optical) disks, phase variation optical disks and the like.
2. Description of Related Art
In the past, when reproducing information from rewriteable optical disks such as MO and phase variation type optical disks and the like, the information was reproduced using a fixed light output that was sufficiently lower than the light output (laser output) used during recording and erasing so as not to accidentally delete or destroy the recorded information. That is, if the reproduction power of the light output during reproduction is Pr, and the maximum value of the reproduction power that does not destroy the information is Prmax, then information was reproduced using a reproduction power Pr such that Pr=fixed (constant) and Pr.ltoreq.Prmax. Thus, a fixed reproduction power less than or equal to Prmax was used.
Meanwhile, in recent years, numerous methods have been conceived in order to increase the recording capacity of optical disks. Fundamentally, it is necessary to make the intervals between the recording tracks narrow and to make the length of the smallest recording marks smaller in order to increase the recording density. In this case, particularly by making the length of the recording marks smaller, the signal amplitude of the reproduced signals corresponding to each mark becomes smaller due to the influence of the OTF (optical transfer function) of the optical system.
The reproduced signals include: amp noise produced by the reproduction amplifier, strength variation noise from the hard disk media and the semiconductor laser that functions as the light source, and shot noise caused during photoelectric conversion in the detector, etc. Consequently, as the signal amplitude corresponding to the marks has become smaller pursuant to the shortening of the mark length, there is a problem in that the desired signals become buried by the noise, making it impossible to reproduce the signals correctly.
Thus, the signal amplitude of the reproduced signals should be made greater, and the noise should be made relatively lower compared to the signal amplitude. The influence of the noise components that are not dependent on the amount of introduced light, such as, for example, the amp noise, etc., can be made relatively smaller by increasing the amount of light introduced into the reproduced signal detector. For example, if the reproduction power Pr is doubled, provisionally, the signal amplitude will be doubled, but the various types of noise described above will not increase much because they are not dependent on the reproduction power. Thus, the S/N (signal to noise) ratio (the ratio of the noise in relation to the reproduced signals) can be nearly doubled, and it becomes possible to reproduce correctly the signals without receiving the influence of noise.
Usually, the ZCAV (Zone-Cav) format is applied in high-density recording. In the ZCAV format, the entire surface of the optical disk is partitioned into several areas in the radial direction, and recording and reproduction are performed such that the smallest marks in each area are about the same length. By doing this, because the number of revolutions is constant, the recording and reproduction frequency reaches a maximum at the outer perimeter of the optical disk, and a wide frequency band becomes necessary to reproduce the signals.
Meanwhile, because the amp noise of the reproduced signals is thermal circuit noise and has the property of becoming larger as the frequency rises, the noise becomes greater when reproducing an outer perimeter of a disk, which has a high recording and reproduction frequency compared to the inner perimeter, causing the S/N ratio to become worse. Also, the maximum power Prmax that does not destroy the recorded information is dependent on the linear velocity of the light spot on the disk, such that it becomes greater in proportion to roughly 1/2 times the linear velocity. Accordingly, in order to make the smallest marks for recording and reproduction smaller across the entire surface of the disk, reproduction methods have been considered in which the reproduction power Pr that does not destroy the information in the inner perimeter of the disk, which has a smaller Prmax, is made smaller, and the reproduction power Pr is made greater in order to improve the S/N ratio in the outer perimeter of the disk, which has a greater Prmax.
Consequently, with such conventional information reproduction apparatus, there was a problem that, due to changing the reproduction power between the inner perimeter and the outer perimeter, when errors occurred, for example, tracking failures due to various causes, such as when tracking the intended tracks by randomly seeking the tracks, and the like, there was the danger of damaging the data of the inner perimeter by the optical head moving into the inner perimeter while in a state of high reproduction power suitable for the outer perimeter.